The present invention relates to an improved process for continuous casting of aluminum-deoxidized steel.
It is common practice in the steelmaking industry to add a small amount of a strong deoxidizer to molten steel prior to casting in order to reduce the oxygen content and also to help produce a fine grain structure in the cast steel. Aluminum has been used as the deoxodizer for many years now and steels so produced are commonly refer to as "aluminum-deoxidized steel".
As a result of the oxidation process, deoxidized steels usually contain numerous tiny microscopic inclusions which are composed predominantly of refractory oxides, e.g., alumina. These microscopic inclusions may vary in size, type and distribution in both the molten and cast steel. It has been known for sometime that the microscopic inclusions greatly influence the flow of molten steel and in some instances may even cause blockage of tundish nozzles during the casting operation. In the past, this problem has been alleviated by reaming out the clogged nozzles with an oxygen lance or the like.
In recent years, there has been a growing tendency for steelmakers to adopt continuous casting methods in the manufacture of steel. These methods are far more economical when compared to conventional billet casting, for example, and can substantially reduce manufacturering costs. However, continuous casting of deoxidized steels has been seriously limited by the problem of nozzle blockage. It has been found in commercial practice that the entire casting operation must be periodically shut down in order to replace or repair clogged nozzles. This of course defeats the whole purpose of continuous casting methods.
In an article "Steel Flow Through Nozzles: Influence of Deoxidizors," by J. W. Farrell and D. C. Hilty, published in Electric Furnace Proceedings, AIME, Vol. 29, 1971, pages 31-46, it is suggested that the simplest solution to the problem of nozzle blockage is a chemical one in which the melting temperature of the oxide produced by the strong deoxiderizer would be lowered below that of the steel. It is postulated that by modifying the microscopic inclusions in this manner they would remain in solution rather than precipitate during casting as an oxide layer inside the nozzle where the temperature decrease is the most pronounced.
The same authors in a later article "Modification of Inclusions by Calcium," published in Iron and Steelmaker, ISS-AIME, two Volumes, 1975, (May), pages 17-22 and (June), pages 20-27, reported that in the case of aluminum-deoxidized steels, inclusion modification of this kind can be readily obtained by the addition of calcium to the molten steel.
Subsequent articles have been published in this field which describe the benefical effects of adding calcium or calcium-bearing materials to deoxidized steels. See for example the article "Mechanism of Clogging of Tundish Nozzle during Continuous Casting of Aluminum-killed Steel", by S. K. Saxena et al, published in Scandinavian Journal Of Metallurgy, Vol. 7, 1978, pages 126-133. See also the article "Influence of Deoxidation on the Castability of Steel," by K. H. Bauer, published in The Metals Society London, Vol. 3, 1977, wherein it is suggested that calcium may be employed as the sole deoxidizer in place of aluminum. However, there have been no studies made so far directed to the effects of both calcium and the aluminum dixoidizer in the molten steel. For example, it has not been known until now whether calcium when added in amounts which have proven effective in some instances to avoid the problem of nozzle blockage will be equally as effective for the same purposes when added to a dixoidized steel in which the weight proportion of aluminum has been varied. It is known, for example, that the weight proportion of aluminum recovered in an aluminum dexoidized steel may vary in amounts up to about 35 percent or more in commercial production and in some cases as much as about 75 percent.